Morphological, structural, microhardness and corrosion characterisations of electrodeposited Ni-Mo and Cr coatings

Abstract

A resistência à corrosão de eletrodepósitos de Cr e Ni-Mo e a influencia do tratamento térmico na estrutura cristalográfica, na morfologia e nas propriedades de microdureza foram investigados. As caracterizações foram feitas usando as técnicas de microscopia eletrônica de varredura (SEM), difração de raios X (XRD) e energia dispersiva de raios X (EDX). Os ensaios de corrosão foram feitos à temperatura ambiente, em solução de NaCl 10 -1 mol dm -3 e usando a técnica de polarização linear potenciodinâmica. O teor de Mo na camada e a eficiência de corrente aumenta com a concentração do íon molibdato no banho de eletrodeposição, enquanto que a morfologia superficial evolui de rugosa e homogênea para trincada com o aumento do teor de Mo na camada. Os ensaios eletroquímicos de corrosão mostram que os eletrodepósitos de Cr são mais resistentes à corrosão que os eletrodepósitos de Ni-Mo e que todas as camadas estudadas corroem em meio de cloreto. Dentre os eletrodepósitos de Ni-Mo estudados, o de Ni-13Mo foi o que apresentou o comportamento de resistência à corrosão mais nobre. A microdureza do eletrodepósito Ni-13Mo aumenta com o aumento da temperatura de tratamento térmico e está relacionado à formação de fases de Ni, Ni 4 Mo e NiMo, durante o tratamento térmico. Ni-13Mo é um potencial candidato a substituir o Cr em aplicações industriais quando a temperatura de operação for maior que 100 °C e que boas propriedades de microdureza sejam requeridas.The corrosion resistance of electrodeposited Cr and Ni-Mo coatings and the influence of heat treatment on the crystallographic structure, morphology and microhardness properties were investigated here. The characterisations were carried out using scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX) techniques. Corrosion tests were performed at room temperature in 10 -1 mol dm -3 NaCl solutions and by potentiodynamic linear polarization technique. The Mo content in the layer and current efficiency increased with the molybdate ion concentration in the plating solution, while the surface morphology evolved from rough and homogeneous to cracked surface with the increase of the amount of Mo in the layer. The electrochemical corrosion tests showed that the Cr coatings have better corrosion resistance than the Ni-Mo coatings in chloride medium and that all the studied coatings corrode in chloride medium. Ni-13Mo coating has the nobler corrosion behavior among the studied Ni-Mo coatings. The microhardness of the Ni-13Mo coatings increased as the annealing temperature increased which is related with the precipitation of Ni, Ni 4 Mo and NiMo phases during the heat treatment of this coating. Ni-13Mo coating is a potential substitute for chromium coating in industrial applications when operating at temperatures higher than 100 °C and good microhardness properties are required.

Characterisation of electrodeposited and heat-treated Ni−Mo−P coatings

“…The observed spherical nodules are explained as a consequence of the growth of secondary nuclei on top of the first layer that was formed on the substrate surface. It has previously been reported in the literature 10,12 that cracked Ni-Mo electrodeposits were obtained when the Mo content in the layer was higher than 17 at.%. Thus, Figure 3A shows that 1 at.% of P is not enough to avoid the formation of a cracked Ni−Mo−P layer which can compromise the corrosion resistance and hardness properties of this coating.…”

“…9 Recently, we showed that electrodeposited Ni−Mo coatings presented inferior corrosion-resistance and hardness properties in comparison to electrodeposited Cr coatings. 10 However, after heat treatment at temperature higher than 100 °C, these coatings presented superior hardness properties in comparison to the heat-treated Cr coatings. These results indicated that Ni−Mo coatings could be a potential substitutes for chromium coatings in industrial applications in which operational temperatures higher than 100 °C and good hardness properties are required.…”

“…These results indicated that Ni−Mo coatings could be a potential substitutes for chromium coatings in industrial applications in which operational temperatures higher than 100 °C and good hardness properties are required. 10 Another point that supports a possible replacement of the Cr coatings by Ni−Mo coatings is that the industrial production of electrodeposited Ni−Mo coatings could produce environmentally harmless wastewater, since molybdenum is a non-toxic metal for the aquatic environment, 11 while the industrial chromium electroplating process requires the use of carcinogenic Cr 6+ ions. However, the electrodeposition of Ni−Mo coatings is limited to a maximum of 17 at.% of Mo in the layer, since electrodeposits with Mo content higher than 17% were cracked, which compromised the corrosion-resistance and hardness properties of the Ni−Mo coatings.…”

“…However, the electrodeposition of Ni−Mo coatings is limited to a maximum of 17 at.% of Mo in the layer, since electrodeposits with Mo content higher than 17% were cracked, which compromised the corrosion-resistance and hardness properties of the Ni−Mo coatings. 10,12 Thus, the challenge in the research of electrodeposition of Ni−Mo-based coatings is to obtain electrodeposits that are non-cracked, even with Mo content in the layer higher than 17 at.%.…”

“…The amorphous character of an electrodeposited coating is usually achieved by the codeposition of the elements of the iron group (Fe, Ni and Co) with an element, that can be a metal such as Mo and W or a metalloid such as P or B, which provokes defects in the crystal lattice of the coating, leading to the absence of a crystallographic structure in the electrodeposited coating. 8,10,12,[17][18][19][20][21][22] Among them, P is the more usual element to be codeposited with the elements of the iron group because it improves the corrosion-resistance properties of the electrodeposited coatings due to the fact that in aqueous medium this element produces a protective surface film formed by the phosphate anion, which kinetically limits the dissolution of the amorphous coatings such as Ni−P and Co−P electrodeposits. 22 Another important requirement to obtain amorphous electrodeposits is the composition of coating.…”